Optical Path Difference Formula for Thin - Film Interference (Equal Thickness Interference)

Formula: Δ L = 2n₂ d cosγ + (λ)/(2) or Δ L = 2n₂ d cosγ. Here, Δ L is the optical path difference, n₂ is the refractive index of the film, d is the film thickness, γ is the angle of refraction (angle of incidence inside the film), and λ is the wavelength. The addition of λ/2 typically corresponds to the half - wave loss upon reflection from a rarer to a denser medium. Applicable Conditions: The upper and lower surfaces of the film are parallel or approximately parallel, incident light is monochromatic or quasi - monochromatic, observing interference of reflected light. Explanation: This formula is used to analyze equal - thickness interference phenomena such as Newton's rings and wedges. The optical path difference is determined by the film thickness and the angle of refraction.